Using a hex editor to edit an image

• We can actually use hex editors to change image files

• Usually, we will use programs like PhotoPlus or PhotoShop

• But if you use a hex editor, you will appreciate the fundamental details of how images are stored in files

• We will change test.bmp using XVI32

• First, however, we must learn some details of the .bmp format

The .bmp format

• A file in the Microsoft .bmp format can also be known as a .dib file

• A detailed description can be found here http://www.cs.ucc.ie/j.bowen/cs1107/imageFormats/bmp/bitmapStorage.html

and

http://www.cs.ucc.ie/j.bowen/cs1107/imageFormats/bmp/formatDesc.html

• The first of these is a local copy of this Microsoft page:

http://msdn.microsoft.com/en-us/library/dd183391(v=VS.85).aspx

The .bmp format• A .bmp file file has four sections• The File Header contains

general details about the file• The Info Header contains

general details about the image in the file

• The Colour Table describes the palette of colours used in the image

• The Pixel Data specifies the contents of the dots in the picture by specifying, for each pixel, which colour in the palette is used in the pixel

The .bmp format

File test.bmp

File test.bmp

File test.bmp

• The first two bytes of the File Header contain the ASCII codes for BM, as specified

File test.bmp• Bytes three to six of

the File Header contain

46 04 00 00hex

• This number is in little-endian format, so it is really

00 00 04 46hex

• This is equal to 1094dec

• We saw earlier that this is, indeed, the size of the file

File test.bmp• Bytes eleven to fourteen

of the File Header contain

36 04 00 00hex

• This number is in little-endian format, so it is really

00 00 04 36hex

File test.bmp• Bytes eleven to fourteen

of the File Header contain

36 04 00 00hex

• This number is in little-endian format, so it is really

00 00 04 36hex

• This number points to the start of the pixel data in the file

File test.bmp• Bytes one to four of the

Info Header contain

28 00 00 00hex

• This number is in little-endian format, so it is really

00 00 00 28hex

• As the format specifies, this is equal to 40dec

File test.bmp• Bytes five to eight of

the Info Header contain

04 00 00 00hex

• This number is in little-endian format, so it is really

00 00 00 04hex

• This is, indeed, equal to the width of the picture in test.bmp, which is 4px wide by 4 px high

File test.bmp• Bytes nine to twelve of

the Info Header contain

04 00 00 00hex

• This number is in little-endian format, so it is really

00 00 00 04hex

• This is, indeed, equal to the height of the picture in test.bmp, which is 4px wide by 4 px high

File test.bmp• Bytes thirteen to

fourteen of the Info Header contain

01 00hex

• This number is in little-endian format, so it is really

00 01hex

• This is, indeed, equal to the number 1 which is specified in the format

File test.bmp• Bytes fifteen to sixteen

of the Info Header contain

08 00hex

• This number is in little-endian format, so it is really

00 08hex

• So 8 bits are used to represent each pixel, allowing for a palette which contains 256 different colours

• Even though only 2 colours are used in the picture

File test.bmp• Bytes seventeen to

twenty of the Info Header contain

00 00hex

• So the picture is not compressed

File test.bmp• Bytes twenty-one to

twenty-four of the Info Header contain

10 00 00 00hex

• This number is in little-endian format, so it is really

00 00 00 10hex

• Since the image is not compressed, this value seems to be irrelevant

File test.bmp• Bytes twenty-five to

twenty-eight of the Info Header contain

12 0B 00 00hex

• This number is in little-endian format, so it is really

00 00 0B 12hex

• This specifies 2834dec pixels per metre horizontally

File test.bmp• Bytes twenty-five to

twenty-eight of the Info Header contain

12 0B 00 00hex

• This number is in little-endian format, so it is really

00 00 0B 12hex

• This specifies 2834dec pixels per metre horizontally

• This agrees with the fact that PhotoPlus says that 4 pixels equals 0.141 centimetres

File test.bmp• Similarly, bytes twenty-

nine to thirty-two of the Info Header contain

12 0B 00 00hex

• This number is in little-endian format, so it is really

00 00 0B 12hex

• Again, this specifies 2834dec pixels per metre horizontally

• Which agrees with the fact that PhotoPlus says that 4 pixels equals 0.141 centimetres

File test.bmp• Bytes thirty-three to

thirty-six of the Info Header contain

00 01 00 00hex

• Since this is in little-endian, it is really

00 00 01 00hex

• This means that there are 256dec colours in the palette

File test.bmp• Bytes thirty-three to

thirty-six of the Info Header contain

00 01 00 00hex

• Since this is in little-endian, it is really

00 00 01 00hex

• This means that there are 256dec colours in the palette

• Since there are 4 bytes per colour, the palette should occupy 04 00hex bytes

• Which it does, ranging from 0036 to 0435

File test.bmp• Bytes thirty-seven to

forty of the Info Header contain

02 00 00 00hex

• Since this is in little-endian, it is really

00 00 00 02hex

• This means that there are only 2 important colours in the palette

• Which agrees with what we know -- the picture contains only blue and red pixels

File test.bmp• Bytes one to four of Colour

Table are

00 00 FF 00 • The fourth byte is unused

00 00 FF 00, so ignore it • Since the three bytes

00 00 FF are little-endian, they are really

FF 00 00• These represent proportions

of RGB (Red, Green, Blue)• Thus, the first colour in the

palette consists of 255red 0green 0blue

• That is, the first colour in the palette is pure red

File test.bmp• Bytes five to eight of the

Colour Table contain

FF 00 00 00 • Since the three bytes

FF 00 00 are little-endian, they are really 00 00 FF

• Thus, the second colour in the palette consists of 0red 0green 255blue

• That is, the first colour in the palette is pure blue

File test.bmp• Before we look at the

pixel data in the file, let's look at the image and zoom in to see it clearly

File test.bmp• In the Pixel Data, the

bottom row is stored first, the pixel details being given in the order left-to-right

• These four pixels are specified to use colour 01 in the colour table, that is the second colour, that is pure blue

File test.bmp• The second row from the

bottom is stored next, the pixels again in left-to-right order

• These four pixels are specified to use colour 00 in the colour table, that is the first colour, that is pure red

File test.bmp• The third row from the

bottom is stored next, the pixels again in left-to-right order

• These four pixels are specified to use colour 01 in the colour table, that is the second colour, that is pure blue

File test.bmp• The top row is stored

last, the pixels again in left-to-right order

• These four pixels are specified to use colour 00 in the colour table, that is the first colour, that is pure red

Changing test.bmp• Let's use XVI32 to

change the last byte in the file, changing it from 00 to 01

• This byte represents the right-most pixel in the top row of the picture

• We have changed its value, making it refer to the second colour in the Colour Table, which is pure blue

• Let's store the modified picture in a file called test2.bmp

File test2.bmp• When we open

test2.bmp in PhotoPlus, we see that we have, indeed, made the change we intended

• The right-most pixel in the top row of the picture has been changed to blue

Storing the test.bmp image more efficiently• Re-open the original test.bmp• We can store its image more efficiently, by reducing the Colour

Table to contain just the two colours that are used• If we use XVI32 to make the appropriate changes, we get the bytes

shown below -- now only 78dec bytes are needed

• Let's store the modified data in a file called test3.bmp

Viewing image in test3.bmp• When we open test3.bmp in PhotoPlus, we see that the image in

the 78-byte file looks exactly like the image in the 1094-byte file

Changing the aspect ratio of the image• We can easily convert the image in test3.bmp into an 8-by-2 image

instead of a 4-by-4. because both images contain 16 bytes• In XVI32, we need change only the two bytes highlighted below• Let's store the modified data in a file called test4.bmp

Viewing image in test4.bmp• The image has two rows, each eight pixels long

Lines must be padded out to multiples of 4 bytes• Below is a file containing a 5-by-3 image, a 15-pixel image

• Although the image has one pixel less than the previous image, the file is larger - it contains 86 bytes instead of 78

• This is because, although each line has only 5 pixels, each line must be padded out to a multiple of 4 bytes, so 8 bytes are needed for each line

• Below, the padding bytes are coloured light blue - zeros are used for padding here, but it appears that the padding can be anything

• Let's store this 15-pixel image in a file called test15d.bmp

Viewing image in test15d.bmp• The image has three rows, each five pixels long• As we see on next slide, the pixels are as we would predict from XVI32

Viewing image in test15d.bmp• The pixels are as we would predict from XVI32

– Remember that the pixels of the top row appear last in the file

– Remember that colours are represented as Blue-Green-Red-unused

A bug in PhotoPlus

• As we shall see later, there is a bug in the way that PhotoPlus handles .bmp files

• So, first, let's check that the view which PhotoPlus gives of the image in test15d.bmp is correct

• We will look at the image in another image editor and in some browsers

Viewing image in test15d.bmp in MS Paint• The View menu has been tuned to 800% so that we can see detail• The image looks the same as we expect

Viewing image in test15d.bmp in Firefox• The View menu has been used to zoom in so that we can see detail• The image looks the same as we expect

Viewing image in test15d.bmp in Opera• The View menu has been used to zoom in so that we can see detail• The image looks the same as we expect

Viewing image in test15d.bmp in MSIE• Internet Explorer does not display .bmp files itself • Instead, it calls MS Paint as a helper program• As we have already seen, in MS Paint the image looks as expected

Absolutely basic 8-bit depth .bmp format• We have seen that the common image display programs ignore much of the format for 8-bit-

depth .bmp files• The minimum we must do when making an 8-bit depth file is

– assign the fixed values shown below to the bytes marked green

– put the image width and depth in bytes 12-15hex and 16-19hex, respectively

– specify the number of colours in bytes 2E-31hex

– specify the colours in 4-byte blocks from byte 36hex onwards (two colours are shown in the example below)

– specify the pixels immediately afterwards, padding the lines if necessary (16 pixels are shown in the example below)

– put the address of the first pixel specification in bytes 0A-0Dhex

– put the overall file size in bytes 02-05hex